Toric lenses are simple lenses which have for one of their surfaces a segment of an equilateral zone of a torus and consequently have different refracting power in different meridians. A torus is a smooth rounded protuberance on a body part. A toric lens is used to correct astigmatism by the use of a surface of compound curvature. Such lenses have been commercially available for a long period of time. The compound curvature constitutes small variations in thickness in the lens in the optic region to make adjustments according to the prescription of the wearer of the lens to correct the wearer's astigmatism problem. The thickness differences in a contact toric lens in the optic region may vary as little as 0.03 millimeters (mm) yet represent from about 8 to about 30 percent of the depth or thickness of the lens.
Because the toric lens contains a surface of compound curvature, the orientation of the lens relative to the orientation of the eye is important for effective visual correction of the astigmatic problem. Such orientation is a more difficult technical problem in the case of contact toric lenses.
There is a class of bifocal contact lens which selects a predetermined region removed from the lens' central axis to locate the near vision element. These lenses rely on refractive monofocal optics to generate the far and near focal powers. Like the contact toric lens, the bifocal contact lens of this type relies on proper orientation of the lens on the eye to assure efficacy of the lens.
In order for such contact lenses to orient appropriately on the eye, they should have a thickness differential coupled to a sloping wedge-like cross-sectional profile in the top midperiphery of the lens. This involves the generation of sloping surfaces from about the periphery, especially the mid-periphery, of the lens such that as the slope of the lens approaches the optic region, the lens gets thicker. This sloping wedge-like profile in the contact lens functions in concert with eyelid movement over the lens to force the alignment of the lens in the eye such that the corrective toric or bifocal surface is properly aligned for visual benefit and correction. Because of the history of the contact lens art, the techniques has been frequently referred to as "ballasting." Though somewhat inappropriate, that term is used herein to refer to the aforementioned sloping wedge-like profile.
There are a number of ways of ballasting a contact lens. They include placing a thin top peripheral cross-sectionally wedge-like section in the lens with or without a slabbed section at the bottom portion of the periphery of the lens. The optical axis of the lens may or may not provide the common axis of curvature for the front and back surfaces of the lens. When it is not, then the thin top peripheral cross-sectionally wedge-like section in the lens is a prism ballast. A lens which possesses a thin top peripheral cross-sectionally wedge-like section with or without a slabbed section at the bottom portion of the periphery of the lens is termed herein a "thin-sectioned ballasted contact lens."
Thin-sectioned ballasted contact lenses have been used in the ophthalmic industry for correcting the visual orientation of lenses used to correct a number of vision defects such as astigmatism and presbyopia. In order to correct these visual anomalies, the contact lens must maintain a preferential orientation on the user's eye. The thin-sectioned ballasted contact lens cooperates with eyelid movement to maintain the required orientation.
Extraordinarily tight lens dimensional tolerances are required to reliably predict the equilibrium position of a thin-sectioned ballasted contact lens on the user's eye. The consequence of inadequate dimensional conformance is unpredictable lens orientation and, as a result, residual astigmatism in the case of toric lenses and blurry vision in the case of bifocal lenses. The unpredictable orientation greatly compromises the correction of astigmatism in the case of toric soft contact lenses and presbyopia in the case of bifocal soft contact lenses. Consequently, such unpredictable orientation frequently compromises the ultimate visual acuity and stability of vision provided by these modalities of vision correction.
The degree of accuracy required of the manufacturing process is much higher for a ballasted contact lens than a spherical lens as a spherical lens does not need to maintain a preferential orientation on the user's eye. Ballasted contact lenses are routinely manufactured using lathing and cast molding techniques. Lathing processes used to manufacture prism ballasted torics are complex and labor intensive as many cutting and polishing steps are needed to generate the required lens geometry. The cast molding technique simplifies and reduces the labor intensiveness of the manufacturing operation. The complex geometry needed to produce a ballasted toric lens is placed on master molds. The master molds are typically replicated with a thermoplastic using injection molding.
The fabrication of the master molds is complex and labor intensive. However, the master mold can be used to reproducibly generate thousands of injection molded parts. The toric or bifocal contact lens is cast against convex and concave injection molded parts. The process yields lenses which do not require surface polishing. Toric lathing processes, as a result of their complexity and labor intensiveness, yield lenses which are substantially more expensive and less reproducible than cast molded torics.
It is known in the contact lens art that spincasting provides a means for reproducibly manufacturing spherical soft contact lenses at a low cost. However, spincasting introduces technical issues, if used for the manufacture of lens that have an irregular shape, that do not exist with lathing and casting. For example, a toric lens has compond curvature and a bifocal lens where the near correction surface are off-centered, has a multifaceted surface which is another form of a compound curvature. In spincasting, these compound surfaces have to be on the anterior surface of the lens and the posterior should have the requisite corneal cup shape. Thus, the female spincasting mold surface for such compound surfaced lenses would have to incorporate such compound surfaces in its design. In cast molding this poses no problem because the posterior is separately controlled by the presence of a male mold. More of the lens-forming monomers can be fed to the mold and the compression of the two molding surfaces in cast molding assures the integrity of both surfaces. In lathing, the lens button being lathed can have a perfectly shaped corneal cup and lathing away of excess plastic from the anterior surface can be used to provide the compound anterior surface without disturbing the corneal cup surface (posterior surface) of the lens.
A spincast contact lens is difficult to produce with ordered symmetrically aligned surfaces. This can be appreciated from a consideration of the dimensions of a contact lens. The hydrated lens typically had a diameter of about 12 to about 16 mm. The center thickness may range from about 0.13 mm, in the case of a -5 diopter lens, to about 0.35 mm, in the case of a +4 diopter lens. Such lenses taper of in thickness to the feathered edges at the periphery. The amount of resin used in spincasting the lens is extremely small. For example, there is used about 30 microliters of the lens-forming monomers in making each lens.
These facts demonstrate the seeming improbability of using spincasting alone as a method for making lenses possessing a compound anterior surface such as a toric and bifocal lens. The expectation would be that the centrifugation of the small amount of lens-forming monomers in the cavity mold would sweep it across the mold's surface in a manner that the posterior surface would replicate or possess a translation of the anterior surface of the lens. This would result in a corneal bowl shape that also possessed the compound surface that is on the anterior surface. This could only result in the fit of the lens being uncomfortable to the wearer of the lens and adversely affect the corrective capabilities of the lens. On the face of it, spincasting would appear to be a poor choice of processes for making contact lenses with compound surfaces.
Quite surprisingly, it has been determined that spincasting can be used to make a lens with a compound anterior surface and with a corneal bowl, posterior surface, that does not possess a translation of the anterior surface.
An objective of the present invention is to simplify the manufacturing and improve the accuracy of ballasted compound surfaced lenses such as toric and bifocal lenses by employing spincasting. It is objective of the invention to inexpensively produce compound surfaced lenses by spincasting. It is another object of the invention to reproducibly produce compound surfaced lenses by spincasting.